Photoconductive switches are devices which produce electrical pulses in response to illumination by a light source, typically a laser. Such switches comprise a pair of spaced-apart electrically conductive electrodes which are mounted on a photoconductor, e.g., a high resistivity semiconductor. A bias voltage is applied across the electrodes and, when the photoconductor is illuminated, current is passed between the electrodes.
The photoconductor is illuminated with light having an energy greater than the band gap of the photoconductor, which is the energy needed to raise electrons from the valence band to the conduction band, i.e., from a non-conductive and high resistance state to a conductive or low resistance state of the photoconductor.
When uniformly illuminated, the conductivity of the entire photoconductor can be changed in the time in which optical energy is delivered. The light generates electron-hole pairs in the photoconductor. This generation of electron-hole pairs increases the conductivity of the photoconductor, allowing current to flow. The electron-hole pairs are rapidly removed by recombining in the photoconductor material. With these materials, when the optical energy is shut off, the current is immediately terminated. This rapid recombination is known as a short excess carrier lifetime.
In other photoconductor materials, e.g., silicon, the electron-hole pairs do not readily recombine in the photoconductor material. With these materials, e.g., silicon, the current shuts off much more gradually when the optical energy is terminated. Gradual recombination is known as a long excess carrier lifetime. Long excess carrier lifetimes result in slow switching speeds. With some of these materials, a second light source can be used to cause conduction of the electron-holes to a third grounding electrode in the photoconductor material and thus, to obtain high switch speeds.
The operating voltage and current of the photoconductive switches is dependent on the intensity of the illumination from the light source. For power switching applications, i.e., applications requiring voltages of at least 100 volts and currents of at least 10 amperes, the intensity of the light on the photoconductor must be at least 50 picojoules. Solid state, gas, and dye lasers are used to deliver light at such an intensity. Such light sources, however, tend to be bulky and expensive which make the power switches bulky and expensive.